Is there a way to use a point other than the origin as the center for "edit object" "add object"?
Is there a way to use a point other than the origin as the center for “edit object” “add object”?


No, there is not. But yes there is (but not i the way you imply with your question).

Background

How would you tell where this point is?

Your expectation is that the projectile is inside the barrel and it moves out of it. This is correct in real live and the audience (player) should think it is this way. 

But this is a game. It's purpose is entertainment. Therefore you do not care things that are not even visible to the audience (projectile in a barrel is not visible). 

As you noticed having a projectile inside the barrel is not something the physics engine really want to deal with. 

You correctly try to achieve you place the projectile right at the end of the barrel and let it travel from there. The physical effects from the time inside the barrel are just applied (the initial velocity).

Where is the end of the barrel?

Who cares! Just place an emitter (empty) at that location and let it emit the projectile. 


When you parent the emitter to the barrel it will follow any turns and motions.



The whole secret is: the barrel is not the emitter.